Method of recovering oil or gas and treating the resulting produced water

ABSTRACT

A method or process for treating wastewater containing high organics, silica, boron, hardness, and suspended and dissolved solids. The method includes degasifying the wastewater for the removal of dissolved gases and thereafter chemically softening the wastewater. After the chemical softening step, the wastewater is directed through a media filter or membrane which removes additional solids and precipitants. Thereafter the wastewater is directed through a sodium ion exchange that further softens the wastewater. The effluent from the ion exchange is directed through a cartridge filter and the effluent from the cartridge filter is directed through one or more reverse osmosis units. At a selected phase of the process, prior to the wastewater reaching the reverse osmosis unit or units, the pH of the wastewater is raised and maintained such that the pH of the wastewater reaching a reverse osmosis unit is at a pH greater than 10.5.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 12/904,286 filed Oct. 14, 2010, which is acontinuation of U.S. patent application Ser. No. 11/609,659, whichmatured into U.S. Pat. No. 7,815,804, and claims priority to provisionalU.S. Patent Application Ser. No. 61/474,517 filed Apr. 12, 2011. Each ofthese references are expressly incorporated herein by reference.

BACKGROUND OF THE INVENTION

Numerous types of wastewater or produced water have relatively highconcentrations of organics, silica, boron, hardness, suspended anddissolved solids. For example, oil recovery operations produce waterthat includes high concentrations of these contaminants. If suchwastewater or produced water is to be discharged or used in high purityapplications, such as a feed to a boiler or once through a steamgenerator or process water, then there must be a substantial reductionin silica, total hardness, dissolved solids and organics.

Ion exchange processes and reverse osmosis processes have been used fordesalting produced water or wastewater. Some practices involving theoperation of reverse osmosis systems usually maintain a neutral pHcondition, which is a pH of approximately 6-8. In the case of feed waterproduced by oil and gas operations, the recovery across reverse osmosissystems is often limited by scaling due to silica or fouling due toorganics. That is, high concentrations of silica in the feed water tendto scale the reverse osmosis membranes due to the concentration ofsilica exceeding solubility limits. Organics that exceed solubilitylimits also tend to foul the reverse osmosis membranes. Scaling due tosilica and fouling due to organics can cause substantial down time ofthe reverse osmosis unit or units, requiring frequent cleaning,replacement and maintenance. The maintenance is obviously expensive andthe down time is costly and inefficient.

In addition, in the case of produced water, for example, processes aredesigned to remove silica and boron. These contaminants are oftenpresent in the form of weakly ionized salts, sicilic acid and boricacid, and generally reverse osmosis membranes are not efficient inrejecting such weakly ionized salts.

Therefore, there has been and continues to be a need for an economicalprocess for treating wastewater or produced water that reduces foulingdue to organics, reduces scaling due to silica, and which willefficiently reduce the concentrations of silica, organics, dissolvedsolids and hardness in the wastewater or produced water.

SUMMARY OF THE INVENTION

The present invention relates to a method for treating a waste stream orproduced water derived from an oil or gas recovery operation. Producedwater contains organics, silica, hardness, dissolved solids, andsuspended solids. Hardness in the produced water is reduced bychemically softening the produced water. During the softening process,the pH of the wastewater is raised to above 10.5. The wastewater is thendirected to a mixing tank where it is vigorously mixed to cause theformation of crystals therein. Free oil, emulsified oil in some cases,and the crystals are removed from the wastewater with a filtrationmembrane. At least a portion of the reject stream produced by thefiltration membrane is recirculated to the mixing tank. The membraneeffluent is directed to an ion exchange unit where residual calcium andmagnesium hardness is removed therefrom. The effluent from the ionexchange unit is directed to at least one reverse osmosis unit wheredissolved solids, organics and boron are removed therefrom.

Other embodiments of the invention include treating the reverse osmosismembrane effluent in an ammonia polisher and/or an oxidation system.

Additional embodiments of the invention include pretreating the producedwater in a degasification system or a gas flotation system.

Other objects and advantages of the present invention will becomeapparent and obvious from a study of the following description and theaccompanying drawings which are merely illustrative of such invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic illustration of the process of the presentinvention.

FIG. 2 is a schematic illustration of the process of the presentinvention including a double pass reverse osmosis system.

FIG. 3 is a schematic illustration of the process of the presentinvention including an ammonia polisher.

FIGS. 4A-4B are schematic illustrations of the process of the presentinvention including an oxidation system.

FIGS. 5A-5B are schematic illustrations of the process of the presentinvention including a heat exchanger and a cooling tower.

FIGS. 6A-6B are schematic illustrations of the process of the presentinvention including a pretreatment degasification system.

FIGS. 7A-7B are schematic illustrations of the process of the presentinvention including a pretreatment gas flotation system.

DESCRIPTION OF THE INVENTION

The present invention relates to a wastewater treatment process fortreating wastewater or produced water that typically contains organics,silica, boron, dissolved solids and suspended solids. Various typeswastewater may contain these contaminants. For example, in the petroleumindustry, produced water typically includes these contaminants. As usedherein, the term “produced water” means water that is produced alongwith oil or gas in an oil or gas recovery process. Typically, the oil orgas is separated from the water and the separated water is referred toas produced water which is one type of wastewater. The present inventionpresents a process that can be utilized to treat various types and formsof wastewaters including, but not limited to, produced water and coolingtower blowdown.

As described subsequently herein, the process of the present inventionentails chemically softening the wastewater in a process that removeshardness. Thereafter the wastewater is subjected to membrane filtrationand ion exchange softening. The wastewater is then directed through oneor more reverse osmosis units. To prevent scaling and fouling of themembranes in the reverse osmosis units, an antiscalant is mixed with thewastewater upstream from the reverse osmosis units and the pH of thewastewater is preferably maintained above 10.5. The wastewater can befurther treated in an ammonia polisher and an oxidation system. Further,the wastewater can be directed through a heat exchanger so that thetemperature of the wastewater is cooled prior to treatment in thereverse osmosis unit(s). Depending on the quality of the wastewaterprior to chemical softening, the wastewater may be pretreated in adegasification system or a gas flotation system.

In FIG. 1, the wastewater treatment system of the present invention isindicated generally by the numeral 10. The system includes a chemicalsoftening unit 20, a membrane filter 30, an ion exchange filtration unit40, a cartridge filter 50, and at least one reverse osmosis unit 60.

Chemical softening unit 20 includes a series of reactors 21, 23, 25disposed sequentially in relation to one another. Each of the reactors21, 23, and 25 includes a mixer and inlets 22, 24, and 26 respectively.As discussed in more detail herein, Inlets 22, 24, and 26 are used toinject chemicals such as softening reagents into the water held inreactors 21, 23, and 25. Downstream from the reactors 21, 23, 25 isdisposed reactor 27 which includes a vertical tube mixer orcrystallization tank. A pump 28 is operative to pump wastewater fromreactor 27 to the downstream membrane filter 30.

In one embodiment the membrane filter 30 is a ceramic ultrafiltrationmembrane and is used to remove suspended solids and precipitants in thewater passing through the membrane. Typically, ceramic membranes rejectparticles having a size of 0.2 μm or larger. However, ceramic membranescan be designed such that they reject particle sizes as low as 0.03 μm.Subsequently, herein is a general discussion on ceramic membranes andtheir applicability to treating feed water streams having one or more ofthe contaminants discussed here. In another embodiment, the membranefilter 30 is a polymeric membrane. Ceramic membranes are desirable whendealing with water having a very high temperature. For example, ceramicmembranes are desirable when the water passing therethrough has atemperature approaching 300° F.

As will be appreciated from subsequent portions of this disclosure, areject recycle line 32 that extends from the membrane filter 30 to thereactor 27. In addition, there is provided a reject waste line 34 thatextends from the membrane filter 30 to a filter press 36. Wastewaterproduced by the filter press 36 is directed through line 38 back to thechemical softening system 20. Membrane filter 30 produces a permeate oreffluent that is directed from the membrane filter 30 via line 39 whichextends to an ion exchange unit 40. Further, the membrane filterproduces a reject stream, a portion of which is recycled via line 32back to the mixing reactor 27.

Ion exchange unit 40 includes a chemical inlet 41 for regeneration ofthe ion exchange resin upon exhaustion. A recycle line 44 is operativelyconnected between the ion exchange unit 40 and the chemical softeningunit 20. Ion exchange unit 40 is also operatively connected to a wasteline 46 that directs waste produced in the ion exchange unit 40 todisposal. A caustic inlet 45 is disposed downstream from the ionexchange unit 40 and is used to inject a caustic solution into the wastestream produced by the ion exchange unit 40. Ion exchange unit 40 alsoincludes a treated effluent line 43 that extends from the ion exchangeunit 40 to a cartridge filter 50. As will be discussed subsequently, theion exchange unit 40 is utilized to remove residual hardness from thewastewater and, in the case of the embodiment disclosed herein, the ionexchange unit 40 is operated in the sodium mode.

Cartridge filter 50 further filters the water which exits the cartridgefilter 50 through filtered water line 52. An antiscalant inlet 47 isdisposed upstream of the cartridge filter 50, as shown in FIG. 1, whichinjects an antiscalant reagent into the water in line 43. Addition ofthe antiscalant reagent provides a soluble chemical equilibrium forscale forming compounds across the downstream reverse osmosis unit 60.In another embodiment however, the antiscalant inlet can be disposeddownstream of the cartridge filter 50 and upstream from the reverseosmosis unit 60. A pump 54 is operatively interconnected betweencartridge filter 50 and reverse osmosis unit 60. A line 52 directs theeffluent from the cartridge filter 50 to the inlet of the reverseosmosis unit 60.

A reject line 61 extends from the reverse osmosis unit 60 and isoperatively connected to the waste line 46 which directs waste producedin the reverse osmosis unit 60 to disposal. Treated effluent line 62extends from the reverse osmosis unit 60 to a discharge area or to apoint where the treated water is subjected to additional treatment. Acarbon dioxide inlet 63 is disposed downstream from the reverse osmosisunit 60 and is used to inject carbon dioxide into the treated effluent.It is appreciated that the reverse osmosis unit 60 produces a rejectstream that is directed into line 61 and a permeate stream that isdirected into line 62. In this embodiment and in all subsequentembodiments, the treated effluent can be utilized as a water source fora steam generation device such as a boiler or a once through steamgenerator. In one example, the treated effluent is used to generatesteam which is injected into an oil and/or gas bearing formation inorder to extract oil and/or gas therefrom.

As stated above, the wastewater treatment system 10 of the presentinvention can be used to treat various types and forms of influentwastewater streams such as a produced water stream or a cooling towerblowdown stream. FIG. 1 illustrates the present invention being utilizedto treat a produced water stream. In this case, an oil/water mixture ora gas/water mixture is pumped up through an oil or gas-gathering welland the mixture is sent to a separator which, in the case of anoil/water mixture, separates the oil from the water to yield an oilproduct and separated water which is termed produced water. The producedwater becomes the feed stream that is directed into the wastewatertreatment system 10. In the case of a gas/water mixture, the separatorseparates the gas from the water to form a gas product and producedwater. The produced water becomes the feed stream to the wastewatertreatment system 10.

While FIG. 1 discloses one basic embodiment of the wastewater treatmentsystem, it should be appreciated that any of the embodiments for thewastewater treatment system 10 shown in FIGS. 1-7 could be utilized fortreating produced water, cooling tower blowdown, or other waste streams.

With reference to FIG. 2, the wastewater treatment system 10 can furtherinclude a second reverse osmosis unit 70. This embodiment is generallyreferred to as a double pass reverse osmosis system because the filteredwater from the cartridge filter 50 is treated in two sequential reverseosmosis units 60, 70. A caustic inlet 64 is disposed downstream from thefirst reverse osmosis unit 60 and upstream from the second reverseosmosis unit. Caustic inlet 64 is used to inject a caustic solution intothe treated effluent in line 62. A pump 65 is operatively connectedbetween the first reverse osmosis unit 60 and the second reverse osmosisunit 70. Permeate produced by the first reverse osmosis unit 60 ispumped from the first reverse osmosis unit 60 to the second reverseosmosis unit 70 by pump 65.

Second reverse osmosis unit 70 produces a permeate stream and a rejectstream. The reject stream is directed into line 72 and recycled to apoint upstream of the first reverse osmosis unit 60. The permeateproduced by the second reverse osmosis unit 70 is directed into line 74.In this embodiment, a carbon dioxide inlet 63 is disposed downstream ofthe reverse osmosis unit 70 and is used to inject carbon dioxide intothe permeate passing through line 74.

FIG. 3 illustrates an embodiment of the wastewater treatment system 10having the chemical softening unit 20, membrane filter 30, ion exchangeunit 40, first and second reverse osmosis units 60, 70 and an ammoniapolisher 80. In this embodiment, the treated effluent line 74 extendsfrom the second reverse osmosis unit 70 to the ammonia polisher 80.Ammonia polisher 80 includes a chemical inlet line 82 for regenerationof the ion exchange resin upon exhaustion and a recycle line 88 that isoperative to recycle waste from the ammonia polisher 80 to the chemicalsoftening unit 20. Ammonia polisher 80 is also operatively connected tothe waste line 46 that directs waste produced in the ammonia polisher 80to disposal. Ammonia polisher 80 further includes a treated effluentline 84 that extends from the ammonia polisher 80 to a discharge area orto a point where the treated water is subjected to additional treatment.A caustic inlet 89 is disposed downstream from the ammonia polisher 80and injects a caustic solution into the treated effluent in line 84.

FIGS. 4A-4B illustrate an embodiment of the wastewater treatment system10 having the chemical softening unit 20, membrane filter 30, ionexchange unit 40, first and second reverse osmosis units 60, 70, ammoniapolisher 80, and an oxidation system 90. FIG. 4A illustrates thechemical softening unit 20, membrane filter 30, and ion exchange unit40, as described above in FIG. 1. FIG. 4B illustrates the cartridgefilter 50, first and second reverse osmosis units 60, 70, and theammonia polisher 80 as described above in FIGS. 2 and 3B. FIG. 4B alsoincludes the oxidation system 90. In this embodiment, the treatedeffluent line 84 extends from the ammonia polisher 80 to an ultravioletlight unit 92 in the oxidation system 90. Oxidation system 90 includes ahydrogen peroxide (H₂O₂) inlet 91 that injects a H₂O₂ solution into thetreated effluent in line 84 upstream from the ultraviolet light unit 92.Oxidation system 90 further includes an irradiated treated effluent line93 that extends from the ultraviolet light unit 92 to a discharge areaor to a point where the treated water is subjected to additionaltreatment. A sodium bisulfite inlet 94 and a caustic inlet 95 aredisposed downstream from the ultraviolet light unit 92 and inject sodiumbisulfite and a caustic solution, respectively, into the irradiatedtreated effluent in line 93.

FIGS. 5A-5B illustrate an embodiment of the wastewater treatment system10 having the chemical softening unit 20, membrane filter 30, ionexchange unit 40, first and second reverse osmosis units 60, 70, ammoniapolisher 80, oxidation system 90, and heat exchanger 100. FIG. 5Aillustrates the chemical softening unit 20, membrane filter 30, and ionexchange unit 40, as described above in FIG. 1. FIG. 5B illustrates thecartridge filter 50, first and second reverse osmosis units 60, 70,ammonia polisher 80, and oxidation system 90, as shown above in FIGS. 2,3B, and 4B.

In the process depicted in FIGS. 5A-5B, there is provided a heatexchanger 100 that is disposed generally between the ion exchange unit40 and the cartridge filter 50. The heat exchanger cools the effluentproduced by the ion exchange unit 40 prior to the effluent reaching theone or more reverse osmosis units. As seen in FIG. 5B, the coolingmedium or water utilized by the heat exchanger 100 is directed from theheat exchanger 100 through line 104 to a cooling tower 102. There thecooling medium or water is cooled and recirculated back to the heatexchanger 100 via return line 103. From time to time, it may benecessary to provide make-up water for use by the heat exchanger 100.Thus, make-up water is taken from the permeate of the reverse osmosisunit 70, in the FIG. 5B example, and directed through line 108 to thecooling tower 102. Further, from time to time, some of the water isremoved from the cooling tower 102 and directed into the waste line 46.

FIGS. 6A-6B illustrate an embodiment of the wastewater treatment system10 having the chemical softening unit 20, membrane filter 30, ionexchange unit 40, first and second reverse osmosis units 60, 70, ammoniapolisher 80, oxidation system 90, heat exchanger 100, and a pretreatmentdegasification system 110. FIG. 6A illustrates the pretreatmentdegasification system 110. In this embodiment, pretreatmentdegasification system 110 is disposed upstream from the chemicalsoftening unit 20 and includes an acid inlet 112 and a water outlet 114.Acid inlet 112 is disposed upstream from a degasification chamber 113and injects an acidic solution into the feed water prior to waterentering the degasification chamber 113. In one embodiment thedegasification chamber 113 is a forced draft degasifier chamber. Thedegasification system 110 further includes a water outlet 114operatively connected to the degasification chamber 113 and to the firstreactor 21 of the chemical softening unit 20.

FIGS. 7A-7B illustrate an embodiment of the wasterwater treatment system10 having the chemical softening unit 20, membrane filter 30, ionexchange unit 40, first and second reverse osmosis units 60, 70, ammoniapolisher 80, oxidation system 90, heater exchanger 100, and apretreatment gas flotation system 120. FIG. 7A illustrates thepreteatment gas flotation system 120. In the embodiment, thepretreatment gas flotation system 120 is disposed upstream from thechemical softening unit 20 and includes an acid inlet 121 and a ferricchloride (FeCl₃) (for example) inlet 122. Acid inlet 121 and the FeCl₃inlet 122 inject an acidic solution and FeCl₃ into the feed water orwastewater respectively prior to the water entering a gas flotationchamber 124. Gas flotation chamber 124 includes a mixer, a polymer inlet123, and a gas inlet 125. Gas flotation system 120 further includes awater outlet 126 operatively connected to the gas flotation chamber 124and to the first reactor 21 of the chemcil softening unit 20. A recycleline 127 extends from the water outlet 126 to the gas flotation chamber124. A pump 128 is disposed along the recycle line 127 and functions topump water through the recycle line 127 and into the gas flotationchamber 124.

With reference to the specific processes illustrated in the figures,feed waer of wastewater influent is directed to the chemical softeningunit 20. The purpose of the chemical softening process is to reducetotal hardness in the feed water to solubility limits, typically lessthan approximately 55 mg/l as CaCO₃. Further, the softening processcarried out in the chemical softening unit 20 removes at least a portionof the silica from the feed water.

As shown in the figures, a first alkali based reactant is added to thewaterwater in reactor 21 through inlet 22. In one embodiment the firstalkali based reactant is calcium hydroxide (Ca(OH)₂). As Ca(OH)₂ ismixed with the wastewater, carbon dioxide (CO₂), calcium bicarbonate(Ca(HCO₃)₂, and magnesium bicarbonate (Mg(HCO₃)₂ dispersed throughoutthe wastewater react with the Ca(OH)₂ to precipitate some calciumcarbonate (CaCO₃) and some magnesium hydroxide (Mg(OH)₂). Ca(OH)₂ alsoreacts with magnesium sulfate (MgSO₄) and magnesium chloride (MgCl₂) inthe wastewater to precipitate magnesium hydroxide (Mg(OH)₂).

In one embodiment, magnesium oxide (MgO) is also added to thewastewater. MgO can be added to the water prior to the water enteringreactor 21 or MgO can be added to the water while in reactor 21. As MgOis mixed with the water, it is converted into magnesium hydroxide(Mg(OH)₂) which acts as an adsorbent. Mg(OH)₂ in the water, createdthrough the addition of Ca(OH)₂ and/or MgO, adsorbs silica dispersedthroughout the water.

Wastewater is transferred from reactor 21 to reactor 23 where a secondalkali based reactant is added to the water through inlet 24. In oneembodiment, the second alkali based reactant is sodium carbonate(Na₂CO₃). As Na₂CO₃ is mixed with the water, the Na₂CO₃ reacts withcalcium sulfate (CaSO₄) and calcium chloride (CaCl₂) in the water toprecipitate CaCO₃.

The addition of these reagents, for example Ca(OH)₂, MgO, and Na₂CO₃,causes hardness compounds to precipitate from the wastewater streambeing treated. After precipitation, these precipitants can be removed byfiltration such as with the filtration membrane and cartridge filter.

After the wastewater is treated in the reactor 23, the wastewater isdirected to reactor 25 where a third alkali based reactant can be addedto the water through inlet 26. In one embodiment, the third alkali basedreactant is a caustic solution, such as sodium hydroxide (NaOH). Thethird alkali based reactant raises the pH of the water to above 10.5. Asdiscussed in more detail below, it is often preferable to maintain thepH of the water in the range of 10.5 to 11.5.

The above description includes the addition of several alkaline reagentsand a caustic reagent. However, it is noted that the addition of each ofthe above reagents is merely an exemplary embodiment of the presentinvention. Further, the present invention encompasses embodiments inwhich only one or two alkaline reagents and a caustic reagent are addedto the water. For example, it may be effective to add only one alkalinereagent and one caustic reagent to the water to remove hardness from thewater and effectively increase the pH of the water.

After the water has been treated in reactor 23, the water is directed tothe mixing reactor 27 where the wastewater is vigorous mixed in thevertical mixer. Mixing in the mixing reactor 27 causes precipitants togrow larger which makes it easier for the downstream membrane filter 30to reject the precipitants. As discussed below, the reject stream of themembrane filter 30 is recycled via line 32 to the mixing reactor 27. Themixing action of the vertical tube mixer in reactor 27, coupled with therecycle of solids or precipitants from the membrane filter 30, givesrise to a crystallization process where the solids form crystals and themixing action realized in the reactor 27 causes the crystals to growlarger. Note, that in mixing reactor 27, the vertical tube mixerincludes a tube-like structure disposed in the reactor and one or moremixers disposed within the two structures. In mixing the variousreagents and precipitants, wastewater in reactor 27 is induced into thetop of the tube and caused to move downwardly through the tube and outthe bottom of the tube. This continuous mixing action causes thewastewater, along with the precipitants or crystals to move back uptoward the top of the reactor outside of the tube. Thereafter thewastewater re-enters the top of the tube. Thus, this continuous mixingaction causes the wastewater to be drawn down through the tube, out ofthe tube, and up the sides of the reactor and back down through thetube. This type of mixing action promotes an efficient crystallizationprocess where the precipitants tend to grow larger. After the crystalsor precipitants have increased in size, the effluent from the mixingreactor 27 is directed to membrane filter 30 via the pump 28. Typically,the effluent from the mixing reactor 27 is directed through the membranefilter 30 at a pressure of approximately 30 psi to approximately 60 psi.

Membrane filter 30 removes suspended solids and particulates. Thepresent wastewater treatment system and process is effective in treatingproduced water which typically includes fee oil and even emulsified oil.See FIG. 1. The membrane filter 30 is effective to remove both free oiland emulsified oil. Removing free oil and emulsified oil with themembrane filter 30 increases the life of the downstream reverse osmosisunit(s) 60, 70. Generally, manufacturers of reverse osmosis unitssuggest that the reject stream of a reverse osmosis unit should have nomore than 1 ppm of free oil therein. Thus, it is preferred to remove asmuch free oil and emulsified oil from the wastewater or produced waterprior to being treated in a reverse osmosis unit. Thus, in oneembodiment it is desirable for the filtrate produced by the membranefilter 30 to have a total hardness of less than 50 mg/l CaCO₃, a freeoil concentration of less than 0.50 mg/l and a turbidity of less than0.5. As noted above, membrane filter 30 produces a reject stream inwhich at least a portion thereof is directed through recycle line 32 tothe mixing reactor 27 where it mixed with the wastewater in the mixingreactor 27. Another portion of the reject stream is directed to wasteline 34 that directs the reject stream from the membrane filter 30 to afilter press 36. Filter press 36 removes excess water from the rejectstream which is directed to recycle line 38 and directed to the chemicalsoftening unit 20. The remaining solids recovered from the filter press36 are directed to disposal.

Membrane filter 30 also produces a filtrate which is directed to the ionexchange unit 40 through filtrate line 39. As the filtrate passesthrough the ion exchange unit 40, Ca²⁺ and Mg²⁺ in the filtrate areremoved through a cation exchange process. For example, in oneembodiment, the ion exchange unit 40 includes a Na⁺ based cation resin.The Ca²⁺ and Mg²⁺ in the filtrate are exchanged with the Na⁺ in theresin. Moreover, other metal cations present in the filtrate areexchanged with the Na⁺ in the resin. Thus, the ion exchange unit 40further reduces the total hardness of the wastewater and reduces theconcentration of other soluble metals in the water.

As the resin in the ion exchange unit 40 becomes saturated with Ca²⁺ andMg²⁺, and other metal ions, the resin needs to be regenerated tomaintain its effectiveness. To regenerate the resin, an acid solutionadded to the ion exchange unit 40 through inlet 41 is directed throughthe resin. A portion of the waste produced through the resinregeneration is recycled to the chemical softening unit 20 throughrecycle waste line 44. A caustic solution is added to the waste streamin the waste line 44 through inlet 45. This addition reduces the pH ofthe acidic waste stream. Another portion of the waste stream producedthrough resin regeneration is directed to the waste line 46 and sent fordisposal.

The softened effluent from the ion exchange unit 40 is directed from theion exchange unit 40 through line 43 to the cartridge filter 50 whichremoves fine particulates present in the effluent. An antiscalant isadded to the effluent through inlet 47 upstream of the cartridge filter50 to maintain a soluble chemical equilibrium for the scale formingcompounds across the downstream reverse osmosis unit(s) 60, 70. Inprocesses such as described herein, it is difficult to eliminate scalingor fouling of the membranes associated with the reverse osmosis unit(s)by chemical softening and softening in the ion exchange unit 40. Forthat reason, the antiscalant is added to the feed water or wastewaterstream ahead of a first reverse osmosis unit 60. In another embodiment,the antiscalant inlet 47 can also be disposed downstream from thecartridge filter 50 and upstream from the reverse osmosis unit 60. Thefiltered water exits the cartridge filter 50 through filtered water line52 and is directed to a first reverse osmosis unit 60 via a pump 54. Toensure that the downstream reverse osmosis units do not experienceextensive fouling or scaling, in one embodiment it is desirable that thefiltered water from the cartridge filter 50 have a total hardness ofless than 0.20 mg/l CaCO₃, a free oil concentration of less than 0.50mg/l, and a silt density index of less than 4.0.

The first reverse osmosis unit 60 reduces organics, silica, boron, andtotal dissolved solids present in the water. Accordingly, the firstreverse osmosis unit 60 produces a reject stream having a relativelyhigh concentration of contaminants and a treated effluent stream havinga relatively low concentration of contaminants. The reject stream isdirected through reject line 61 to waste line 46 for disposal. Thetreated effluent or permeate is directed through the treated effluentline 62 to a discharge area or to a point where the treated effluent issubjected to additional treatment. In one embodiment, it is desirablefor the treated effluent or permeate for the first reverse osmosis unit60 to have less than 1,000 μS/cm conductivity. Carbon dioxide (CO₂) isinjected into the treated effluent in the treated effluent line 62through inlet 63. The addition of CO₂ decreases the pH of thewastewater. As discussed above, the treated effluent from the reverseosmosis unit 60 has a generally high pH. When CO₂ is added to the waterit dissolves and forms a carbonic acid, H₂CO₃, a generally weak acid.Thus, the addition of CO₂ is beneficial when the treated effluent isdischarged or subjected to downstream treatment and requires a downwardadjustment in pH.

Appearing below under Table 1 is a summary of exemplary data for acooling tower blowdown feed water treated in the process describedabove. Note that calcium hardness is reduced in the chemical softeningunit 20 from 500 ppm to 8 ppm. In addition, magnesium hardness isreduced in the chemical softening unit from 100 ppm to 2 ppm. Further,silica is reduced in the chemical softening unit from 150 ppm to 80 ppm.Again, the pH in the chemical softening process is raised to 10.7. Thetable below refers to free oil, emulsified oil, and soluble oil. As usedherein, if an oil particle is 30 μm or greater, it is referred to asfree oil. If the oil particle is between 1 and 30 μm, it is referred toas emulsified oil. If the oil particle is less than 1 μm, it is referredto as soluble oil.

It is noted that the chemical softening unit 20 does not decrease thetotal alkalinity in the feed water. Rather, the chemical softening unitactually increases the alkalinity in the feed water. For example, inpreliminary tests and as described below, the feed water had a totalalkalinity of 150 ppm as CaCO₃. After treatment in the chemicalsoftening unit, the effluent had a total alkalinity of 400 ppm as CaCO₃.A decrease in total alkalinity only occurs after treatment in the ROsystem. Maintaining a high alkalinity in the water increases therejection rate of organics in the water passing through the RO system.

TABLE 1 Cooling Tower Blowdown Chemical Ceramic Ion Feed SofteningMembrane Exchange RO System Treated Water Effluent Filtrate EffluentPermeate Effluent pH (S.U.) 6.5-8.5 10.7 10.7 10.7 10.2 7.5 TotalAlkalinity 150 400 400 400 <75 <75 (ppm as CaCO₃) Ca-Hardness 500 8.08.0 0.08 Non-detect Non-detect (ppm as CaCO₃) Mg-Hardness 100 2.0 2.00.02 Non-detect Non-detect (ppm as CaCO₃) Dissolved 150 80 80 80 <0.5<0.5 Silica (ppm) Free Oil (ppm) 10.0 10.0 <0.2 <0.2 Non-detectNon-detect Emulsified Oil 5.0 5.0 <0.2 <0.2 Non-detect Non-detect (ppm)Soluble Oil 10.0 10.0 10.0 10.0 <0.5 <0.5 (ppm) TSS (ppm) 30 1,000 <0.2<0.2 Non-detect Non-detect TDS (ppm) 4,000 4,500 4,500 4,500 150 150Boron (ppm) 5.0 5.0 5.0 5.0 <0.5 <0.5 Total Organic 30 30 30 30 <0.5<0.5 Carbon (ppm)

In another embodiment, shown in FIG. 2, a double pass reverse osmosissystem is employed. In this embodiment, the treated effluent is directedto a second reverse osmosis unit 70 that further reduces organics,silica, boron, and total dissolved solids present in the water. Whenusing a double pass reverse osmosis system, a pH adjustment of thetreated effluent may be required between the two reverse osmosis unitsin order to facilitate increased rejection of the boron remaining in thewater. Accordingly, a caustic solution can injected into the treatedeffluent through inlet 64 which is disposed downstream from the firstreverse osmosis unit 60 and upstream from the second reverse osmosisunit 70. The amount of caustic injected is calculated so as to maintainthe pH greater than 10.5 and preferably to a pH of 11.

Second reverse osmosis unit 70 also produces a reject stream having arelatively high concentration of contaminants and a permeate or treatedeffluent stream having a relatively low concentration of contaminants.The reject stream is directed through recyclable waste line 72 to thefirst reverse osmosis unit 60. CO₂ is injected into the treated effluentthrough inlet 76 disposed downstream from the second reverse osmosisunit 70. As mentioned above, the addition of CO₂ decreases the pH of thewater and thus is beneficial if the treated effluent is being dischargedor subjected to downstream treatment which requires a downwardadjustment in pH. FIG. 2 shows that the treated effluent is directedthrough the treated effluent line 74 to a discharge area or to a pointwhere the treated effluent is subjected to additional treatment. In oneembodiment, it is desirable for the treated effluent or permeate fromthe second reverse osmosis unit 70 to have less than 75 μS/cmconductivity and a boron concentration of less than 0.83 mg/l.

The present process aims to control the pH of the wastewater passingthrough the one or more reverse osmosis units 60, 70 above 10.5.Maintaining a high pH of the water substantially reduces organic foulingand silica scaling of the membranes in the reverse osmosis units.Further, the solubility of organics generally increases with pH. Forexample, at a pH over 10 the solubility of organics is approximately 350mg/l. However, at a pH of 6 the solubility of organics is just above 50mg/l. The same relationship holds true for the solubility of silica. Forexample, the solubility of silica at a pH of about 10.5 is almost 900mg/l. However, at a pH of 8 the solubility of silica is about 100 mg/l.By maintaining the pH of the feed water above 10.5, these particularscaling and fouling contaminants are maintained in solution and can berejected by the one or more reverse osmosis units 60 or 70 withoutscaling or fouling.

The process described above can also be modified to include otherprocesses. For example, as shown in FIG. 3, the treated effluent fromthe double pass reverse osmosis system is directed to the ammoniapolisher 80 via treated effluent line 74. Ammonia polisher 80 includes acation exchange resin. As the treated effluent passes through the cationexchange resin, the H⁺ in the cation exchange resin are exchanged withNH₄ ⁺ in the treated effluent. The treated effluent exiting the reverseosmosis unit 70 may, in some cases, have an ammonium hydroxide (NH₄OH)concentration that can be toxic to the environment if discharged.However, after NH₄ ⁺ are exchanged for H⁺ in the ammonia polisher 80,the treated water exiting the ammonia polisher 80 has a significantlysmaller concentration in NH₄OH. In one embodiment, it is desirable forthe ammonia polisher 80 to reduce the ammonia in the water to less than0.3 mg/l as N.

As the resin in the ammonia polisher 80 becomes saturated with NH₄ ⁺,the resin needs to be regenerated to maintain its effectiveness. Toregenerate the resin, an acid solution is added to the ion exchange unit40 through inlet 82. A portion waste produced through resin regenerationis recycled to the chemical softening unit 20 through recyclable wasteline 88. Another portion of the waste stream produced through the resinregeneration is directed to the waste line 46 and sent for disposal.After the water is treated in the ammonia polisher 80, a causticsolution is added to the treated water in the treated water line 84through inlet 89 disposed downstream from the ammonia polisher 80. Theaddition of the caustic solution is preferred if the treated waterexiting the ammonia polisher 80 has a generally low pH. Typically,caustic solution is added to the treated water to increase the pH of thetreated water to between approximately 6.5 and approximately 9 in orderto meet environmental regulations. Then, the treated water in treatedwater line 84 is directed toward a discharge area or to a point wherethe treated water is subjected to additional treatment.

In the embodiment shown in FIGS. 4A-4B, the treated water is directed toan oxidation system 90 through the treated water line 84. A hydrogenperoxide solution is injected into the treated water through inlet 91.Treated water and the hydrogen peroxide (H₂O₂) solution is then directedto the ultraviolet light unit 95 where it is irradiated with ultravioletlight. Irradiation by ultraviolet light converts the H₂O₂ in the waterinto hydroxyl radicals (HO.). These radicals oxidize the organic contentin the water. In one embodiment it is desirable for the oxidation system90 to remove the carbon biological oxygen demand (CBOD) in the water toa concentration of less than 25 mg/l.

The irradiated water exits the ultraviolet light unit 92 through line 93and is then injected with a sodium bisulfite solution through inlet 94.The sodium bisulfite solution removes residual H₂O₂ from the water. Theirradiated water can also be injected with a caustic solution throughinlet 95. The addition of a caustic solution is preferred if theirradiated water has a generally low pH. As described above, the causticsolution is added to the water to increase the pH of the water tobetween approximately 6.5 and approximately 9 in order to meetenvironmental regulations. Then, the irradiated water in line 93 iseither directed toward a discharge area or to a point where the treatedwater is subjected to additional treatment.

In another embodiment, shown in FIGS. 5A-5B, the temperature of thetreated effluent exiting the ion exchange unit 40 is cooled through theuse of a heat exchanger 100 and a cooling tower 102. In typical producedwater applications, the produced water influent has a temperature ofapproximately 160° F. to approximately 200° F. It is beneficial to coolthe water prior to treatment in the reverse osmosis unit(s).Accordingly, treated effluent in line 43 is directed to the heatexchanger 100 where it is cooled to a temperature of approximately 95°F. Cooling water is directed from the cooling tower 102 to the heatexchanger 100 through line 103. Cooling water from the heat exchanger100 is then directed back to the cooling tower 102 via line 104. Aportion of the cooling water can also be directed from cooling tower 102to the waste line 46 and sent for disposal.

Further, after the treated effluent is treated in the first and secondreverse osmosis units 60, 70, a portion of the treated water from thesecond reverse osmosis unit 70 is directed to the cooling tower 102through line 108. This water is used as make-up cooling water for use inthe heat exchanger 100.

Appearing below under Tables 2 and 3 is a summary of exemplary data foroil field produced water treated in the process described above. Notethat calcium hardness (expressed as CaCO₃) was reduced in the chemicalsoftening unit 20 from 400 ppm to 8 ppm. In addition, magnesium hardness(expressed as CaCO₃) was reduced in the chemical softening unit from 100ppm to 2 ppm. Further, silica was reduced in the chemical softening unitfrom 300 ppm to 80 ppm. The pH in the chemical softening process wasraised to 10.7. Again, it is noted that the chemical softening unit doesnot decrease the total alkalinity in the feed water. Rather, thechemical softening unit actually increases the alkalinity in the feedwater. For example, in the preliminary test described below, the feedwater had a total alkalinity of 300 ppm as CaCO₃. After treatment in thechemical softening unit, the effluent had a total alkalinity of 400 ppmas CaCO₃. A decrease in total alkalinity only occurs after treatment inthe RO system. Maintaining a high alkalinity in the water increases therejection rate of organics in the water passing through the RO system.

TABLE 2 Oil Field Chemical Ceramic Ion Produced Softening MembraneExchange Water Effluent Filtrate Effluent pH (S.U.) 6.5-8.5 10.7 10.710.7 Total Alkalinity 300 400 400 400 (ppm as CaCO₃) Ca-Hardness 400 8.08.0 0.08 (ppm as CaCO₃) Mg-Hardness 100 2.0 2.0 0.02 (ppm as CaCO₃)Dissolved 300 80 80 80 Silica (ppm) Free Oil (ppm) 100.0 100.0 <0.2 <0.2Emulsified 10.0 10.0 <0.2 <0.2 Oil (ppm) Soluble Oil (ppm) 100.0 100.0100.0 100.0 TSS (ppm) 50 1,000 <0.2 <0.2 TDS (ppm) 7,000 7,500 7,5007,500 Boron (ppm) 26.0 26.0 26.0 26.0 Total Organic 100 100 100 100Carbon (ppm) Total Ammonia (ppm) 20.0 20.0 20.0 20.0

TABLE 3 Ammonia Advanced RO System Polisher Oxidation Treated PermeateEffluent Effluent Effluent pH (S.U.) 10.2  4.5  6.5  7.5 TotalAlkalinity <75   Non- 20   20   (ppm as CaCO₃) detect Ca-Hardness Non-Non- Non- Non- (ppm as CaCO₃) detect detect detect detect Mg-HardnessNon- Non- Non- Non- (ppm as CaCO₃) detect detect detect detect Dissolved<0.5 <0.5 <0.5 <0.5 Silica (ppm) Free Oil (ppm) Non- Non- Non- Non-detect detect detect detect Emulsified Non- Non- Non- Non- Oil (ppm)detect detect detect detect Soluble Oil (ppm) <0.5 <0.5 <0.5 <0.5 TSS(ppm) Non- Non- Non- Non- detect detect detect detect TDS (ppm) 150  <10   <20   <20   Boron (ppm) <0.5 <0.5 <0.5 <0.5 Total Organic <0.5<0.5 Non- Non- Carbon (ppm) detect detect Total Ammonia (ppm) 20.0 <0.1<0.1 <0.1

Other embodiments of the present invention include pretreatments of thefeed water prior to treatment in the chemical softening unit 20. Forexample, in FIG. 6A, the feed water is subjected to a degassing processin a degasification system 110 prior to treatment in the chemicalsoftening unit 20. The degasification process is particularly useful forfeed waters containing volatile organic carbons and dissolved gases. Insuch cases, an acid is injected through inlet 112 and mixed with thefeed water to partially convert the bicarbonates present in the feedwater to CO₂ and to maintain hydrogen sulfide or other dissolved gasesin a gaseous state. Gases present in the water are pulled from thesystem through the degasifier and scrubbed using activated carboncanisters. In one embodiment, the degasification process utilizes aforce draft degasifier or DO_(x) stripper to reduce the CO₂ and thehydrogen sulfide present in the feed water. Other types of degasifierssuch as vacuum, membrane or depurator type degasifiers can also be usedfor this application. Typically, the pH is lowered to a range of 4.5 to6.5 ahead of the degasifier and the effulent from the degasifier istypically in the pH range of 5.0-7.0. Degassed water exits thedegasification system 110 through outlet 114 and is then directed to thechemical softening unit 20, where it undergoes treatment describedabove.

The present invention may also include a pretreatment gas flotationsystem 120. For example, in FIG. 7, the feed water is subjected to gasflotation prior to treatment in the chemical softening unit 20.Typically gas flotation systems are useful for removing free oil fromthe feed water and reducing turbidity and the organic concentration inthe feed water. In one embodiment, the water is dosed with an acidicsolution through acid inlet 121 and FeCl₃, for example, through inlet122. Typically, the FeCl₃ dosage for COD removal is between 100-200mg/l. After the feed water has been injected with an acidic solution andFeCl₃, the feed water is directed into gas flotation chamber 124. Apolymer is then added to and mixed with the water in gas flotationchamber 124 through inlet 123. A gas, such as nitrogen (N₂) or methane(CH₄), is directed through the water in the gas flotation chamber 124through inlet 125. The gas bubbles facilitate the removal of free oiland insoluble organic particulates, which rise to the top of the waterwhere they are skimmed off as waste. The treated water exits the gasflotation system 120 through outlet 126. A portion of the treated wateris directed to the chemical softening unit 20, where it undergoestreatment as described above. The remainder of the water exiting the gasflotation system 120 is directed through recycle line 127 back to thegas flotation chamber 124 via pump 128.

It is further noted that the present invention is not limited to thespecific combination of elements described above or shown in thedrawings. Rather, the present invention encompasses embodiments that donot include all of the above elements. For example, the embodimentsshown in FIGS. 6 and 7 do not necessarily require the use of an ammoniapolisher, an oxidation system, and/or a heat exchanger. Likewise, theembodiment shown in FIG. 5 does not necessary require the use of anammonia polisher and/or an oxidation system.

However, each of the embodiments in the present invention includes amembrane filtration unit. As described above, the membrane filtrationunit can comprise a ceramic membrane. Specific details of ceramicmembranes are not dealt with herein because the details of suchmembranes not per se material to the present invention, and further,ceramic membranes are known in the art. For a review of general ceramicmembrane technology, one is referred to the disclosures found in U.S.Pat. Nos. 6,165,553 and 5,611,931, the contents of which are expresslyincorporated herein by reference. These ceramic membranes, useful in theprocesses disclosed herein, can be of various types. In some cases theceramic membrane may be of the type that produces both a permeate streamand a reject stream. On the other hand, the ceramic membranes may be ofthe dead end type, which only produces a permeate stream and fromtime-to-time the retentate is backflushed or otherwise removed from themembrane. For example, when treating produced water, the ceramicmembrane may require cleaning by back pulsing the permeate through themembrane. However, when the transmembrane pressure across the ceramicmembrane reaches a substantially high pressure, such as betweenapproximately 40 psi and 45 psi, it may be desirable to perform a“clean-in-place” of the ceramic membrane.

The structure and materials of the ceramic membranes as well as the flowcharacteristics of ceramic membranes varies. When ceramic membranes areused to purify produced water, the ceramic membranes are designed towithstand relatively high temperatures as it is not uncommon for theproduced water being filtered by the ceramic membranes to have atemperature of approximately 90° C. or higher.

Ceramic membranes normally have an asymmetrical structure composed of atleast two, mostly three, different porosity levels. Indeed, beforeapplying the active, microporous top layer, an intermediate layer with apore size between that of the support, and a microfiltration separationlayer. The macroporous support ensures the mechanical resistance of thefilter.

Ceramic membranes are often formed into an asymmetric, multi-channelelement. These elements are grouped together in housings, either asingle element in a housing or multiple elements in a housing, and thesemembrane modules can withstand high temperatures, extreme acidity oralkalinity and high operating pressures, making them suitable for manyapplications where polymeric cannot be used. Several membrane pore sizesare available to suit specific filtration needs covering themicrofiltration, the ultrafiltration, and nanofiltration ranges from 1micron down to 250 Dalton MWCO).

Ceramic membranes today run the gamut of materials (from alpha aluminatosilicon carbide). The most common membranes are made of Al, Si, Ti orZr oxides, with Ti and Zr oxides being more stable than Al or Si oxides.In some less frequent cases, Sn or Hf are used as base elements. Eachoxide has a different surface charge in solution. Other membranes can becomposed of mixed oxides of two of the previous elements, or areestablished by some additional compounds present in minor concentration.Non-oxide membranes are also available such as silicon nitride orsilicon carbide with silicon carbide membranes being most prevalent. Lowfouling polymeric coatings for ceramic membranes are also available.

Ceramic membranes are typically operated in the cross flow filtrationmode. This mode has the benefit of maintaining a high filtration ratefor membrane filters compared with the direct flow filtration mode ofconventional filters. Cross flow filtration is a continuous process inwhich the feed stream flows parallel (tangential) to the membranefiltration surface and generates two outgoing streams.

A small fraction of feed called permeate or filtrate, separates out aspurified liquid passing through the membrane. The remaining fraction offeed, called retentate or concentrate contains materials rejected by themembrane.

The separation is driven by the pressure difference across the membrane,or the trans-membrane pressure. The parallel flow of the feed stream,combined with the boundary layer turbulence created by the cross flowvelocity, continually sweeps away particles and other material thatwould otherwise build up on the membrane surface.

The process of the present invention has many applications. The processcan be used in oil field produced water treatment for surface dischargeor for use in steam generating devices to generate steam for use in oilrecovery. Likewise, the process can be used in gas field produced watertreatment for surface discharge. In addition, the process of the presentinvention can be used to treat refinery wastewater for reuse with zeroliquid discharge. Moreover, the process or the present invention hasapplications for treating cooling tower blowdown as well as FGD scrubberblowdown. Still further, the process of the present invention hasapplications in treating industrial wastewater, such as automotivewastewater.

The invention claimed is:
 1. A method of recovering oil or gas from anoil or gas well and treating resulting produced water, comprising: a.recovering an oil/water or gas/water mixture from the oil or gas well;b. separating oil from the oil/water mixture or separating gas from thegas/water mixture to produce an oil or gas product and the producedwater, and wherein the produced water includes organics, silica,hardness, dissolved solids and suspended solids; c. reducing hardness inthe produced water by chemically softening the produced water in achemical softening unit which results in the formation of hardnessprecipitants; d. raising the pH of the produced water to 10.5 and above;e. directing the produced water having said pH of 10.5 and above to amixing tank forming a part of the chemical softening unit and mixing theproduced water in said mixing tank and causing crystallization of theprecipitants within the produced water; f. after crystallization,removing free oil and crystals from the produced water by directing theproduced water through a membrane filter and producing a membraneeffluent and a membrane reject stream containing free oil and thecrystals; g. recirculating at least a portion of the membrane rejectstream to the chemical softening unit and causing the crystals in rejectstream to grow larger; h. directing the membrane effluent to an ionexchange unit and further softening the membrane effluent by removingresidual calcium and magnesium hardness therefrom and producing an ionexchange effluent and a waste stream; i. after further softening themembrane effluent, directing the ion exchange effluent to at least onereverse osmosis unit and removing dissolved solids from the ion exchangeeffluent and producing a reject stream and a permeate stream whichcontain ammonium hydroxide (NH₄OH); j. reducing the concentration ofammonium hydroxide in the reverse osmosis permeate stream by directingthe reverse osmosis permeate stream through an ammonia polisher locateddownstream from the reverse osmosis unit; k. regenerating the ammoniapolisher by adding an acid solution to the ammonia polisher whichgenerates a waste stream produced by the ammonia polisher; l. directingat least a portion of the waste stream produced by the ammonia polisherto the chemical softening unit and subjecting the waste stream from theammonia polisher to treatment in the chemical softening unit; m. whereinthe ammonia polisher produces a treated effluent; n. mixing hydrogenperoxide (H₂O₂) with the treated effluent from the ammonia polisher; o.after mixing hydrogen peroxide with the treated effluent, subjecting theammonia polisher effluent to irradiation and converting the hydrogenperoxide in the treated effluent into hydroxyl radicals (HO.); p.employing the hydroxyl radicals to oxidize organic content contained inthe treated effluent from the ammonia polisher; and q. after irradiatingthe treated effluent from the ammonia polisher, mixing sodium bisulfitewith the treated effluent from the ammonia polisher to remove residualhydrogen peroxide.
 2. The method of claim 1 wherein the produced waterincludes free oil and wherein the method includes, prior to removinghardness, directing the produced water to a treatment unit disposedupstream of the chemical softening unit for removing free oil andreducing turbidity and the concentration of organics in the producedwater and including: mixing an acid and a coagulant with the producedwater; after mixing the acid and the coagulant with the produced water,directing the produced water to a gas flotation chamber; mixing apolymer with the produced water in the gas flotation chamber; directinga gas through the produced water in the flotation chamber wherein gasbubbles associated with the gas to facilitate the removal of the freeoil and insoluble organic particulates in the produced water as the gasbubbles rise to the top of the produced water in the gas flotationchamber and the method includes skimming the gas bubbles from theproduced water; and directing the produced water from the gas flotationchamber and directing at least a portion of the produced water to thechemical softening unit and recycling another portion of the producedwater from the gas flotation chamber back to the gas flotation chamber.